The present invention relates to an optical system for an optical pick-up that is capable of using a plurality of kinds of optical discs whose cover layers have different thickness.
The optical disc includes an information layer on which digital information is recorded, and a transparent cover layer that covers the information layer. A laser beam from the optical pick-up is converged to form a beam spot on the information layer through the cover layer. A turntable of an optical disc apparatus rotates the optical disc mounted thereon, and the optical pick-up, which is movable along a radial direction of the optical disc, reproduces the recorded signal from the optical disc or records the information onto the optical disc. Difference of thickness of the cover layer changes the position of the information layer with reference to the turntable, which changes the distance between the optical pick-up and the information layer.
Namely, the thicker the cover layer is, the farther the distance to the beam spot from the optical pick-up is. For example, since the cover layer of a compact disc (CD) or that of a CD-R has the thickness of 1.2 mm, and the thickness of the cover layer of a digital versatile disc (DVD) is 0.6 mm, the optical pick-up is required to move the beam spot away from the optical pick-up by 0.6 mm in the cover layer (0.4 mm in air) when the DVD Is replaced with the CD or the CD-R.
There are two methods to move the beam spot along an optical axis direction. The first method changes the divergence of the incident laser beam onto the objective lens, which is equivalent to change an object distance. For instance, the change of the distance between the laser source and a collimator lens changes the object distance. The second method moves the objective lens along the optical axis while keeping the incident laser beam parallel.
In the first method, spherical aberration sharply varies in undercorrected direction as the divergence becomes larger (the object distance decreases), which disturbs wave front of the laser beam. Thus, the diameter of the beam spot increases, which prevents the optical disc apparatus from reproducing the recorded information from the optical disc. In this connection, since the cover layer is a plane parallel plate disposed in a convergent laser beam, it changes the spherical aberration in overcorrected direction as the thickness thereof increases. However, since the variation of the spherical aberration in the undercorrected direction due to the divergence change far exceeds the variation in the overcorrected direction due to the thickness change, the spherical aberration in the undercorrected direction remains as a result.
The second method can be accomplished using a focusing actuator. An optical pick-up is provided with the focusing actuator to move the objective lens along the optical axis to keep the laser beam in focus. Therefore, if the focusing actuator has a driving stroke that is longer than 0.4 mm, the optical pick-up can move the beam spot In response to the change of the cover layer thickness. However, since the large stroke tends to increase an inclination of the optical axis of the objective lens, a complex mechanism is required to prevent the inclination, which increases the cost.
It is therefore an object of the present invention to provide an optical system of an optical pick-up, which is capable of moving a beam spot along an optical axis in accordance with thickness of the cover layer without moving an objective lens.
For the above object, according to the present invention, there is provided an improved optical system of an optical pick-up that is capable of using a first optical disc having a first cover layer and a second optical disc having a second cover layer thicker than the first cover layer, which includes a light source portion that emits a first laser beam having a first wavelength for the first optical disc and a second laser beam having a second wavelength longer than the first wavelength for the second optical disc, and an objective lens that converges the first laser beam from the light source portion onto an information layer of the first optical disc through the first cover layer and that converges the second laser beam from the light source portion onto the information layer of the second optical disc through the second cover layer. The objective lens is provided with a diffractive lens structure that has a plurality of concentric ring-shaped steps.
The optical system of the present invention further satisfies the following requirements (a), (b) and (c).
(a) The light source portion emits the laser beam such that divergence of the second laser beam incident on the objective lens is larger than divergence of the first laser beam.
(b) The diffractive lens structure has a plurality of concentric ring-shaped steps to have wavelength dependence such that spherical aberration varies in the overcorrected direction as wavelength of incident light increases.
(c) The objective lens converges the first and second laser beams of an identical diffractive order while keeping a constant distance between the objective lens and the surfaces of the first and second cover layers.
The different divergence of the incident laser beam onto the objective lens as the requirement (a) changes a paraxial beam spot according to the thickness of the cover layer. However, the spherical aberration in the undercorrected direction becomes large for the second optical disc with larger divergence, only when the divergence becomes larger. In order to cancel the spherical aberration caused by the divergence change, the diffractive lens structure has the wavelength dependence as described in the requirement (b).
In this construction, the switch from the first laser beam to the second laser beam increases the divergence and the wavelength thereof. The lager divergence moves the paraxial beam spot away from the light source portion and changes the spherical aberration in the undercorrected direction. On the other hand, the larger wavelength varies the spherical aberration in the overcorrected direction by means of the diffractive lens structure. As a result, the switch of the laser beam moves the beam spot along an optical axis without increasing the spherical aberration.
Further, the use of the identical diffractive order beams as the requirement (c) allows the diffractive lens structure to be optimized for a single diffractive order, which enables to maximize diffractive efficiency with the effective use of light amount.
An additional optical path length added by the diffractive lens structure is expressed by the following optical path difference function "PHgr"(h):
"PHgr"(h)=(P2h2+P4h4+P6h6+ . . . )xc3x97xcex
where P2, P4 and P6 are diffractive coefficients of second, fourth and sixth orders, h is a height from the optical axis and xcex is wavelength of incident light.
It is preferable that the diffractive lens structure satisfies the following condition (1);
7 less than "PHgr"(h45)/xcexxe2x88x92P2xc3x97(h45)2 less than 15 xe2x80x83xe2x80x83(1)
where h45 is the height from the optical axis of a point where a light ray whose NA is 0.45 intersects the diffractive lens structure.
When the thickness of the first cover layer is 0.6 mm and the thickness of the second cover layer is 1.2 mm, the firs wavelength xcex1 and the second wavelength xcex2 preferably satisfy the following condition (2);
0.81 less than xcex1/xcex2 less than 0.85.xe2x80x83xe2x80x83(2)
Further, the blaze wavelength xcexB of the diffractive lens structure satisfies xcex1 less than xcexB less than xcex2 at least in a central area close to the optical axis. More preferably, the blaze wavelength xcexB of the diffractive lens structure in the central area satisfies the following conditions (3) and (4);
0.87 less than xcexB/xcex2xe2x80x83xe2x80x83(3)
xcexB/xcex1 less than 1.13.xe2x80x83xe2x80x83(4)
Still further, the additional optical path length added by the ring-shaped step of the diffractive lens structure in a peripheral area may be shorter than that in the central area. In particular case, the diffractive lens structure is only formed within a central area of a lens surface of the objective lens, and a peripheral area thereof is formed as a continuous surface. At least an area outside an 85% line of an effective diameter of the objective lens is defined as the peripheral area. The boundary between the central area and the peripheral area may be an 80% line of the effective diameter.
The light source portion preferably emits the laser beams such that the first laser beam is incident on the objective lens as a parallel beam and the second laser beam is incident on the objective lens as a divergent beam.